The invention pertains to an improved method of welding together sheets utilizing high energy lasers, to form layered sheet structures for use in the aircraft industry.
Superplastic forming combined with diffusion bonding (SPF/DB) is finding increased usage in sandwich structures in the aircraft industry (see U.S. Pat. No. 3,927,817 entitled "Method of Making Metallic Sandwich Structures" by Hamilton, et. al. which is incorporated herein by reference). Superplasticity is the capability of certain metals to develop unusually high elongations with reduced tendency towards necking, within a limited temperature and strain rate range. Diffusion bonding is a metallurgical joining of similar metallic parts which are pressed together at elevated temperatures and pressures.
Many of the same alloys used in superplastic forming can also be used in diffusion bonding. When the two processes are combined, the temperatures and pressures for both processes are similar so that complex and expanded sandwich structures can be formed in what is essentially a one-step operation.
However, SPF/DB has several limitations:
1. only those materials which are superplastic may be used, PA1 2. the structures must be raised to high superplastic forming temperatures and pressures, PA1 3. the considerable stretching may produce a non-uniform product, having non-uniform strength properties, and PA1 4. certain materials cannot be readily diffusion bonded.
A novel process for fabricating sandwich structures without the use of superplastic materials, is described in U.S. Pat. No. 4,361,262 entitled "Method of Making Expanded Sandwich Structures" and in U.S. application, Ser. No. 466,987 filed 2-16-83, entitled "Accordion Expansion Process" both by Leonardo Israeli, which are both incorporated into this specification by reference. The process is essentially an unfolding process, and usually requires minimal tensile stretching of the material during expansion, i.e. the expansion of the structure is due substantially to unfolding rather than stretching. The accordion expansion process may be used as an alternative for superplastic forming. However, diffusion bonding, requires high temperatures and pressures, and is limited to certain materials. Hence, an alternative bonding process is needed that does not require high temperatures and pressures, that is applicable to a broad range of materials, and that can be used with accordion expansion to form sandwich structures.
Lasers are well suited as a manufacturing tool. Material processing is currently one of the most important industrial applications of lasers. Laser welding, which can be accomplished at or near atmospheric conditions, produces the highest energy concentration of all welding processes. Lasers can generate a high power density that is localized and controllable over a small area. Also, lasers allow for cost efficient energy utilization, minimal distortion and softening in the surrounding interlayer, and simplified material handling. Since lasers result in the application of considerable amounts of high energy in short time intervals, high speed manufacturing, accuracy, and repeatability are inherent in laser applications.
What is needed is a process that will utilize the many advantages of laser processing to weld sheet structures prior to expansion to achieve the same kind of monolithic sandwich structures produced by superplastic forming and diffusion bonding.